1. Field of the Invention
This invention relates generally to an air delivery sub-system for a fuel cell system and, more particularly, to an air delivery sub-system for a fuel cell system, where the air delivery sub-system includes a bi-directional mass flow meter (MFM) that measures airflow to and from a turbomachine type compressor in the sub-system to detect compressor surges.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perflurosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode charge gas that includes oxygen, and is typically a flow of forced air from a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product.
FIG. 1 is a plan view of a fuel cell system 10 including an air delivery sub-system 12 and a fuel cell module (FCM) 14 having a fuel cell stack of the type discussed above. The sub-system 12 includes a turbomachine compressor 16 that provides charge air to the cathode side of the FCM 14. The compressor 16 can be any suitable turbomachine type compressor, such as a centrifugal, radial, axial, mixed flow, etc., compressor. This type of compressor is desirable in the system 10 because it is low cost and low weight, and operates with low noise as compared to the positive displacement compressors, such as twin-screw compressors, that are currently employed in fuel cell systems. The hydrogen fuel input to the FCM 14 is not shown in this diagram. Cathode exhaust, including unused air and water, is emitted from the FCM 14 through a cathode exhaust line 26. A back pressure valve 24 in the cathode exhaust line 26 is opened and closed to control the pressure within the FCM 14, and thus, control stack pressure, membrane humidity, etc.
A motor 18 drives the compressor 16 at the appropriate speed to provide the desired amount of charge air to the FCM 14 for the desired output power. Air from the environment is filtered by a filter/attenuator 20 that also reduces compressor whine. The filtered air is sent through a mass flow meter (MFM) 22 that measures the airflow through the compressor 16. A signal indicative of the airflow through the compressor 16 from the MFM 22 is sent to a controller 28. The controller 28 controls the speed of the motor 18 to control the airflow through the compressor 16 to provide the proper air stoichiometry or lambda. The controller 28 also controls the orientation of the back pressure valve 24 to control the pressure within the FCM 14, and thus, membrane humidity. Many factors determine the speed of the compressor 16, including desired output power, ambient temperature, altitude, etc.
It is necessary that the compressor 16 operate on its map of pressure ratio (outlet pressure/inlet pressure) versus air flow. This map of pressure ratio is bound by a surge line at which the compressor 16 suffers from an audible flow reversion caused by excessive backpressure as a result of the stack pressure within the FCM 14. This backpressure is generally caused by the back pressure valve 24. In other words, the pressure within the FCM 14 sometimes causes a reverse flow of air through the compressor 16 that is determined by the drive power from the motor 18, the altitude and the temperature. The map of the pressure ratio is also bound by a choke line where the maximum airflow is reached with minimal pressure for a given compressor speed.
The compressor 16 cannot operate at pressure ratios that put the compressor 16 into a surge condition because of severe oscillation of the airflow through the compressor 16 that could damage the compressor 16. Therefore, the system 10 requires a surge protector that identifies a reverse airflow through the compressor 16 to prevent compressor surge. A reverse airflow through the known positive displacement compressors did not present a problem or cause compressor damage, and thus, surge detection is typically not required on known fuel cell systems.